1. Field of the Invention:
This invention relates in general to earth boring machines, particularly shaft drills that typically use a hydraulic system, electrically controlled, for moving the drill stem and a large diameter drill bit. The invention especially relates to those components of such machines used to make-up and break-out the drill stem sections when inserting or removing drill stem from the bore hole.
2. Description of the Prior Art:
There are commercially available machines which utilize hydraulic cylinders to forcibly move in a linear, upward or downward direction a power swivel, which is commonly driven by a hydraulic motor and pump. When adding or subtracting a drill stem member to the total string of such members, the power swivel is rotated while back-up means hold a portion of the drill string to cause make-up or break-out of a selected, threaded connection of the drill stem. The power swivel is threaded to connect to an upper end of a drill stem memmber, and a back-up device must engage and hold the drill stem member while the power swivel is rotated such that its threads engage and mate with those of the drill stem. The prior art includes a number of machines falling within this general description. There still remains, however, a need for better solution to some of the problems existing with these machines. There is a need, for example, for an improved system that assures in a simple but reliable manner that the lower connection of a drill stem, and not the connection with the power swivel, will break-out when removing the drill stem from the bore hole. Further, there is a need for improvement of the means used to grip and engage the drill stem for back-up so that the entire drill stem is prevented from rotating while making-up or breaking-out a selected connection.